Preheat the oven to 350°F. Lightly grease a 9-inch springform pan with melted butter and line the bottom with nonstick paper.

Quarter, core and peel the apples, then slice thinly. Put the slices in a bowl and pour the lemon juice over.

Put the eggs, sugar and lemon zest in a bowl and whisk with a handheld mixer until the mixture is thick and mousse-like. When the whisk is lifted out, the mixture should hold its trail for a few seconds.

Sift half the flour, all the baking powder and the salt over the egg mousse, then fold in gently with a large metal spoon. Slowly drizzle in the melted butter from the side of the bowl and fold in gently with the spoon. Sift over the remaining flour, fold in gently, then add the apples and fold these in equally gently.

Spoon into the pan and level the surface. Bake for 40 minutes, or until a skewer comes out clean. Let it settle in the pan for about 10 minutes, then invert on a wire rack. Turn the cake the right way up and sprinkle vanilla sugar over the top. Decorate with the citrus zest. Serve warm, with the goat's milk cream.